1. Field of the Invention
The present invention relates to digital comb filters and more particularly, to a digital comb filter for use in a video cassette recorder for controlling the rotation speed and phase of rotating magnetic heads.
2. Description of the Related Art
A drum servo apparatus is installed in a home video cassette recorder (VCR) to control the rotation speed and rotation phase of a rotary disc on which magnetic heads are mounted. The disc is driven by a drum motor. A frequency generator (FG), which generates a signal (FG signal) having a frequency proportional to the rotation speed of the drum motor, and a pulse generator (PG), which generates a pulse signal (PG signal) when the rotation of the disc reaches a predetermined phase, are fixed to the drum motor. The rotation speed and phase of the disc are controlled by regulating the drive current of the drum motor responsive to the FG signal and PG signal. Specifically, control of the rotation speed of the disc is implemented by controlling the rotation speed of the drum motor so that the period of the FG signal corresponds to a predetermined target value. Control of the rotation phase of the disc is implemented by controlling the rotation phase of the drum motor so that the phase of the PG signal has a predetermined relationship with a reference signal.
Unfortunately, however, the amplitude and phase of the FG signal does not remain constant even if the rotation speed of the drum motor remains constant. It is because of the effects of fixing error, eccentricity of the rotation axis or the electromagnetic field of the motor, etc., that parts of the FG are fixed around the rotation axis of the drum motor. This state is called FG unevenness. FG unevenness causes a phenomenon whereby the frequency of the FG signal is not constant even when the rotation of the drum motor is constant, and therefore the rotation of the drum motor is varied conversely.
As a method of solving this problem, a method of removing the component of an integer times the number of rotations of the drum motor was reported in "Television Gakkai Technical Report", Vol. 12, No. 17. FIG. 1 is a block diagram of a digital comb filter according to the above report. Referring to this figure, drum motor 10 has an associated frequency generator (FG) 12 which generates a pulse signal (FG signal) having a frequency proportional to the rotation speed of drum motor 10. According to a disclosed embodiment, FG 12 outputs four pulse signals during each rotation of drum motor 10.
Pulse detector 14 is connected to the output of FG 12, for shaping the waveform of the FG signal. Period measurement circuit 16 is connected to the output of pulse detector 14 for generating a difference signal which represents the difference between the periods of a target signal and input pulses.
Subtracter 18 is connected to the output of period measurement circuit 16. Subtracter 18 subtracts the deviational signal which will be described later, from the difference signal generated by period measurement circuit 16. Both adder 20 and drum motor 10 are connected to the output terminal of subtracter 18. Adder 20 adds an accumulation signal, which will be described later, to the output signal of subtracter 18. Accumulation circuit 22 having four registers, 221 through 224, connected in series, is connected to the output of adder 20. Accumulation circuit 22 stores the four differences between each target period and each input FG pulse period to four registers 221 through 224, respectively.
Coefficient circuit 24 is connected to the output of accumulation circuit 22. Coefficient circuit 24 multiplies the output from accumulation circuit 22 by a coefficient K where K is in the following range: 0&lt;K&lt;1. The other input terminal of adder 20 is connected to the output of register 224 of accumulation circuit 22. The other input terminal of subtracter 18 is connected to the output of coefficient circuit 24.
The operation of the above digital comb filter will now be described below.
An FG signal is generated from FG 12 by the rotation of drum motor 10. The output waveform of the FG signal is shaped by pulse detector 14. In period measurement circuit 16, the difference signal representing the differences between the periods of a target signal and the output from pulse detector 14 is generated. The output signal of coefficient circuit 24, described in more detail below, is subtracted from the difference signal by subtracter 18.
The output of subtracter 18 is added to the output of register 224 of accumulation circuit 22 by adder 20. The output of adder 20 is first stored in register 221 of accumulation circuit 22. When the next output from adder 20 is generated, the output stored in register 221 is transferred to register 222, and the new output of adder 20 is stored in register 221. Each time a new output of adder 20 is stored in register 221, the output currently stored in registers 221 through 223 is transferred sequentially to the next register. The output of register 224 is transferred back to adder 20 and to coefficient circuit 24. Through this process, the four most recent outputs from adder 20 will be stored in registers 221 through 224. In this case, because four FG signals are generated by a rotation of drum motor 10, a group of four difference signals corresponding to a rotation of drum motor 10 are accumulated in accumulation circuit 22. Thus, in subtracter 18, the last group of four difference signals, which are accumulated in accumulation circuit 22, are sequentially multiplied by K of coefficient circuit 24 and subtracted from the four difference signals corresponding to the next rotation.
For example, if the period of the FG signal measured first in period measurement circuit 16 is different from the target period, a difference signal is obtained from period measurement circuit 16. If the contents of register 224 is zero, the difference signal from period measurement circuit 16 is output to drum motor 10 and adder 20 without a change of value from subtracter 18. In this case, unevenness of the FG signal is not removed. However, in the next rotation period of drum motor 10, the above difference signal will have been transferred to register 224 through adder 20 and registers 221 through 223. Thus, in subtracter 18, the difference signal from period measurement circuit 16 corresponding to the first FG signal in the next rotation of drum motor 10 is subtracted from the difference signal stored in register 224 multiplied by coefficient K. The value from subtracter 18 is thereby reduced and in every period thereafter, difference signals are reduced. Finally, the difference signal reaches zero, and the unevenness of the FG signal generated is removed by the above circuit. Such output from subtracter 18 is supplied to drum motor 10.
The unevenness of the rotation caused by noise, etc., results in the difference signal removing such noise at the output of subtracter 18. Thus, the signal for reducing the unevenness is supplied to motor 10.
The unevenness of the rotation caused by noise, etc., results in the difference signal removing such noise at the output of subtractor 18. Thus, the signal for reducing unevenness is supplied to motor 10.
In the above digital comb filter, if coefficient K of coefficient circuit 24 is adequately small, the reduction of the gain characteristic in the low frequency band may be small. However, in the initial state, such as when the rotation of drum motor 10 begins, the time needed to reduce unevenness of the FG signal is long.
A large coefficient K in coefficient circuit 24 may be used to decrease response time. However, if large coefficient K is adopted, the noise compensating gain characteristic of the low frequency band, (that is, near direct current) is reduced. FIG. 2 illustrates a characteristic of the digital comb filter shown in FIG. 1, and shows this problem.